This invention relates generally to a method and apparatus for producing a pH change in a solution. More specifically, the invention relates to producing a pH change in a solution by irradiating the solution with visible light. With greater specificity, but without limitation thereto, the invention relates to using light to alter the pH of a solution to thereby cause an expansion and/or contraction of a pH dependent polymer immersed in the solution.
There exist a number of natural and synthetic fibers and gels that are expandable and contractible in volume when activated by an environmental change, such as exposure to a change in solvent composition, temperature, pH, electric field or photo irradiation, for example. As a commercially exploitable technology, the fibers and gels have applications in many fields, such as, for example, use in sensors, switches, motors, pumps, non-metallic operations and use in the medical and robotic fields where it is envisioned that these materials will be able to carry out the function of human muscle tissue.
The work of W. Kuhn and B. Hargitay as described in xe2x80x9cMuskelahnliche Arbeitsleistung Kunstlicher Hochpolymerer Stoffexe2x80x9d, Z. Elektrochemie 1951, 55(6), 490-502, incorporated by reference herein is one example of a synthesized polymer material capable of expansion and/or contraction. When the Kuhn and Hargitay polyacrylamide fiber, known as polyacrylic acid-polyvinyl alcohol (PAA-PVA), is placed within a solution of appropriately increasing pH, a 10% increase in fiber length is claimed to be observed.
Similarly, the work of T. Tanaka, D. Fillmore, S-T. Sun, I. Nishio, G. Swislow, and A. Shah described in the article xe2x80x9cPhase Transitions in Ionic Gelsxe2x80x9d Phys. Rev. Lett. 1980, 45(20), 1636-1639, incorporated by reference herein discloses an observed 400% volume collapse for a polyacrylamide gel disposed in a 50% acetone-water solvent mixture in which the pH of the solvent is lowered at constant temperature and solvent composition.
The work of Kuhn and Hargitay as well as Tanaka and Fillmore et al use a typical approach to changing the pH of a solution. In this approach, the pH is changed by manually dripping an acid or base into the solution. This technique, known as the xe2x80x9cacid dripxe2x80x9d method, relies upon the rate of the diffusion of hydrogen ions to a polymer site and is considered undesirably slow for certain polymer applications, such as use in synthetic muscles.
Besides the pH activation method of Kuhn and Hargitay and Tanaka and Fillmore et al, there exist electrical polymer activation schemes in which p-electron conjugated conducting polymers and electronically doped non-conducting polymers are electrically activated (expanded and contracted). An example of this activation method has been characterized by Shahinpoor et al as described in the article of D. J. Segalman, W. R. Witkowski, D. B. Adolf, and M. Shahinpoor titled: xe2x80x9cTheory and Application of Electrically Controlled Polymeric Gelsxe2x80x9d, Smart Materials and Structures, Vol. 1 (no. 1), M. V. Gandhi and B. S. Thompson (Eds.), London: Chapman and Hall, 1992, 95-100. Like the pH activation method described above, the Shahinpoor et al method depends on the slow diffusion of ions to the active site of a polymer and therefore is also considered too slow for certain polymer applications such as use in synthetic muscles.
In addition to the activation approaches described above, there exist optical activation methods for causing volume changes in polymer fibers and gels. Noteworthy of these is the work of M. Irie and D. Kunwatchakun described in xe2x80x9cPhotoresponsive Polymers. 8. Reversible Photostimulated Dilation of Polyacrylamide Gels Having Triphenylmethane Leuco Derivativesxe2x80x9d, Macromolecules 1986, 19(10), 2476-2480. The Irie-Kunwatchakun studies were among the earliest on photoinduced volume changes in polymer gels. Photosensitive molecules, such as leucocyanide and leucohydroxide, were incorporated directly into a polymer""s network. Irradiation with UV light produced a 2.2-fold reversible dimension change, but no significant volume change (phase transition) took place in the polymer studied, as the UV light-induced pH change was far from the pH null point of the polymer gel. Thus the magnitude of the dimension change was not optimized for certain applications such as robotics.
In the work of researchers Mamada and Tanaka as described in A. Mamada, T. Tanaka, D. Kungwatchakun, and M. Irie in xe2x80x9cPhotoinduced Phase Transition of Gelsxe2x80x9d, Macromolecules 1990, 23, 1517-1519 and as described in A. Mamnada, T. Tanaka, D. Kungwatchakun, and M. Irie in U.S. Pat. No. 5,242,491 titled: xe2x80x9cPhoto-induced Reversible, Discontinuous Volume Changes in Gelsxe2x80x9d and issued Sep. 7, 1993, photoinduced phase transitions in gels were observed. The copolymer used was that of Irie-Kunwatchakun described above. At a given temperature, the polymer gel discontinuously swelled in response to UV irradiation and shrank when the UV light was removed. It is hypothesized that this swelling is due to dissociation into ion pairs, thereby increasing internal osmotic pressure within the gel. The shrinking process of this method is governed by ion diffusion and recombination, making the speed of the reverse process impossible to control, thereby hindering its usefulness in many polymer actuator applications.
In either of the UV studies described above, the UV radiation can cause undesired ionization, photolysis and molecular ligation of a utilized polymer.
Finally, in the work of A. Suzuki and T. Tanaka described in the article xe2x80x9cPhase Transition in Polymer Gels Induced by Visible Lightxe2x80x9d, Lett. Nature 1990, 346, 345-347, visible light was used to irradiate a gel containing a light-sensitive chromophore located in the backbone of an expandable and contractible copolymer. The chromophore absorbed the light and the light energy was then dissipated locally as heat by radiationless transitions, the result of which increased the xe2x80x9clocalxe2x80x9d temperature of the polymer. Unlike the UV studies, the polymer expansion is a rapid process and is due to the direct heating of the polymer network by light. Yet the process of returning the polymer to its original size requires cooling, which becomes increasingly difficult as the temperature of the surrounding solution approaches the temperature of the polymer. This reverse process is too slow for many polymer uses such as in synthetic muscles.
Because many reactions are based on either acid or base catalyzations, including those of the polymers described above, researchers have investigated various approaches to promoting rapid pH changes. Such has been the case of Anthony Campillo et al as described in the article by A. J. Campillo, J. H. Clark, R. C. Hyer, S. L. Shapiro, K. R. Winn, and P. K. Woodbridge titled: xe2x80x9cThe Laser pH Jumpxe2x80x9d, Proc. Intl. Conf. Lasers ""78, Orlando, Fla., Dec. 11-15, 1978, Chem. Phys. Lett. 1979, 67(2), 218-222; the article by A. J. Campillo, J. H. Clark, S. L. Shapiro, K. R. Winn, and P. K. Woodbridge, titled: xe2x80x9cExcited-State Protonation Kinetics of Coumarin 102xe2x80x9d, Chem. Phys. Lett. 1979, 67(2), 218-222; the article by J. H. Clark, S. L. Shapiro, A. J. Campillo, K. R. Winn, titled: xe2x80x9cPicosecond Studies of Excited-State Protonation and Deprotonation Kinetics. The Laser pH Jumpxe2x80x9d, J. Am. Chem. Soc. 1979, 101(3), 746-748; and U.S. Pat. No. 4,287,035 issued to John H. Clark, Anthony J. Campillo, Stanley L. Shapiro, and Kenneth R. Winn on Sep. 1, 1981.
The work of Campillo et al relies on excited-state proton transfer reactions to change the [H+] of a solution by several orders of magnitude. Campillo et al used a picosecond spectroscopy tool to directly measure excited-state deprotonation-protonation reaction rate constants. To promote a pH change, a UV laser with a pulse width of 20 picoseconds was used to excite 2-naphthol-6-sulfonate to a higher (S1) electronic state. From the measured rate constants, Campillo et al determined that the excited-state pKa value was 1.9, as opposed to the ground-state value of 9.1. This 7-unit change in pKa corresponds to a 7-order of magnitude increase in the acid dissociation constant, Ka. Campillo""s findings are consistent with earlier studies which show that excited-state Ka values can differ from ground-state values by many orders of magnitude, see the disclosure of J. F. Ireland and P. A. H. Wyatt titled: xe2x80x9cAcid-Base Properties of Electronically Excited States of Organic Moleculesxe2x80x9d, Adv. Phys. Org. Chem. 1976, 12, 131-221.
Campillo et al claim that a major use of their technique is initiation of acid-base catalyzed ground-state reactions. For example, the reactants A and B are present in solution at pH 7. The ground state reaction, A+Bxe2x86x92C, occurs only at pH 4. By exciting the Campillo et al xe2x80x9cjump moleculexe2x80x9d, 2-naphthol-6-sulfonate, a subnanosecond jump from pH 7 to pH 4 can be achieved, thereby enabling the desired ground-state reaction. Referring to FIG. 1, a schematic state energy level diagram illustrates the path by which the xe2x80x9cjump moleculexe2x80x9d 2-naphthol-andsulfonate travels to produce the pH change described. The 2-naphthol-6-sulfonate is irradiated with LTV light and is excited from ground state S0 to first excited singlet state S1. Radiative decay (florescence) then occurs bringing the molecule back to its ground state.
A major drawback of the Campillo technique is the extremely short duration of the accompanying pH change, typically 10 nanoseconds. While Campillo proposes that the excited state duration, and hence pH change, could be prolonged through use of repetitious irradiation, such an irradiation would require a bombardment of photons on the order of a million times a second. An additional shortcoming of the Campillo technique, when utilized with expandable and contractible polymers such as those described above, is that the utilized UV radiation promotes undesirable polymer ionization, photolysis and other molecular ligation. Additionally, the extremely narrow illumination path (0.1 mm or 5D-6 cubic centimeters) provided by the utilized 266 nanometer laser is considered insufficient to effectively illuminate an expandable/contractible polymer to undergo an appreciable change in volume.
The invention provides a method and apparatus of rapidly changing the pH of a solution by way of a pH jump molecule that is activated by visible light. An application of the present invention is the ground-state reaction of changing the volume of an expandable and contractible polymer for simulated muscle applications as well as for other applications.
To permit these applications, it is desirable (1) to use a source of excitation energy that is not harmful to a utilized polymer; (2) to produce an in-situ pH change in which hydrogen ions become rapidly present at a polymer site; (3) to sustain the resultant pH change long enough and in a volume large enough for desired ground-state reactions to occur, for example, the fully reversible expansion and contraction of a polymer; and (4) to provide a mechanism for efficient dissipation of heat produced as a result of the source of excitation energy.
Candidate pH xe2x80x9cjump moleculesxe2x80x9d considered suitable for providing sufficient polymer actuation (activation) should possess the following characteristics:
(1) the jump molecules should have long lifetimes at room temperature, e.g 10 milliseconds;
(2) the jump molecule acidity constants should be grossly different in ground and triplet states, e.g., 7 orders of magnitude;
(3) the resultant pH change should go through the midpoint (pH null point) of the utilized polymer; and
(4) either the non-protonated or the protonated form of the jump molecule should absorb in the visible region of the spectrum.
In accordance with the present invention, an apparatus and method incorporating these desirable features are disclosed. The invention includes a pH jump molecule that permits visible light excitation to provide a long lasting pH change to a pH dependent polymer or other pH driven reactant. The attendant pH change occurs rapidly (in nanoseconds) and will last for the excited state lifetime of the jump molecule. Further irradiation by either a continuous wave or appropriately pulsed laser can sustain the pH change indefinitely. The pH jump molecule is phosphorescence. That is, heat resulting from the light activation is efficiently discharged by radiative decay through room temperature phosphorescence lifetimes existing on the order of milliseconds. Thus an expandable and contractible polymer can be made to respond rapidly to a change in pH while the radiant heat-release mechanism of the invention allows the polymer to return to its initial configuration in a millisecond time frame, suitable for a variety of useful applications, including robotics.
Accordingly, it is an object of this invention to provide a method and apparatus for producing a rapid pH change in a solution.
A further object of this invention is to produce a rapid pH change in a solution that is useful in causing the expansion and/or contraction of a polymer.
Another object of this invention is to produce a rapid pH change in a solution that lasts long enough and is prevalent enough to be useful in causing the expansion and/or contraction of a polymer.
Still another object of this invention is to produce a rapid pH change in a solution that is useful in causing the expansion and/or contraction of a polymer while minimizing damage to the polymer.
Still yet another object of this invention is to produce a rapid pH change a solution by irradiating the solution with visible light.
Yet another object of this invention is to produce a pH change in a solution by irradiating the solution with visible light in which any heat produced by the light is rapidly dissipated.
Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.